JERUSALEM (CNN) -- A truce meant to allow humanitarian aid into war-torn Gaza crumbled minutes after it began Wednesday, while Israeli and Palestinian diplomats considered talks that could lead to a more lasting cease-fire.

Flames rise into the sky after an Israeli airstrike Wednesday night in Rafa, on the Gaza-Egypt border.

The truce was part of Israel's stated desire to open a "humanitarian corridor" into Gaza in response to mounting concerns over shortages of food, water and medicine in the territory since Israeli airstrikes began December 27.

But minutes after the three-hour truce was set to begin at 1 p.m., Israeli forces fired at militants in the Hamas-ruled Palestinian territory.

Israel has said Hamas, the Palestinian movement that controls Gaza, smuggles weapons from Egypt into Rafah on the Gaza side through underground tunnels. Shutting down that supply route is one goal of Israel's nearly two-week military operation against Hamas. Learn what's behind the conflict »

At least 680 Palestinians have been killed and 3,000 wounded in Gaza since the campaign began on December 27, Palestinian medical officials said. Nearly a third of the deaths and 45 percent of the injuries are women and children, according to the U.N.'s Office for the Coordination of Humanitarian Affairs. Seven Israeli soldiers and three civilians have been killed.

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The three-hour truce is scheduled to take place every other day, Lerner said, but humanitarian agencies say the timeframe is not long enough to alleviate the "deepening humanitarian crisis" in Gaza.

"We are feeding 750,000 [people] on a permanent basis," U.N. Relief and Works Agency spokesman Chris Gunness said. "We need to build on this three-hour window; we need to expand that window and let it lead to a permanent cease-fire."

Israeli and Palestinian diplomats were preparing for meetings in Egypt to discuss a Gaza truce plan that still was being formulated.

Israel will send two envoys to Cairo, Egypt, in the coming days to discuss the plan, an Israeli foreign ministry official said Wednesday. Palestinian diplomatic officials said Palestinian Authority President Mahmoud Abbas, who was in Spain on Wednesday, was planning to go to Cairo. Watch CNN's Christiane Amanpour report on diplomatic efforts »

The Israeli military said Hamas militants were firing mortars from the school, prompting an artillery strike that killed at least 40 people and wounded dozens more. U.N. officials said there were no militants at the school.

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"We are 99.9 percent certain that there were no militants. There were no militant activities in the school or in the school compound," said Gunness of the U.N. Relief and Works Agency.

The Palestine Red Crescent said five ambulances had been hit by Israeli forces, killing six people. The agency did not give a breakdown of paramedics and patients, and CNN was not able to independently verify these allegations.